In the preparation of packages of paper or cardboard sheet, it is common to provide sheet stacks which can be provided on pallets and, for a stack change, i.e. when a previously formed stack is completed, to engage a portion of the stack with a clamping device. In this manner, a stack of sheets is clamped on one side to prevent shifting of this stack or portion or the shifting of individual sheets thereof during the stack change process. Clamping devices of this type can include two parallel clamping beams which can engage a stack of sheets or a plurality of such stacks side by side across the machine width to thereby hold the sheets against relative movement.
In EP-B-0 316 568, an apparatus is described for stacking sheets having a clamping device of this type. This apparatus allows the removal of a full stack without interrupting the feed of the sheets to the stacking location, the continuity building of a new stack, and the deposit of the clamped portion of the stack onto the stacking platform once the previously-formed stack has been removed. The apparatus operates also without diverting the on-coming sheets from the stacking location. For this purpose, the apparatus makes use of an auxiliary stacking platform which can be inserted into the stacking region to hold the newly-formed stack while the previously-completed stack therebelow is transported away. To allow insertion of the auxiliary stacking platform between the finished stack and the paper sheets thereabove without damaging the sheets or shifting the sheets in the newly-forming stack, prior to the insertion of the auxiliary stacking platform, the portion of the stack above the completed stack is engaged between tongues of two clamping elements on one side of the stack. In this system, the stack formed above the insertion plane is gripped and held during the insertion operation.
The clamp drive in this case utilizes double-acting pneumatic piston-and-cylinder units disposed at opposite ends of the upper clamping beam, and which allows the upper clamping beam to move upwardly and downwardly relative to the lower clamping beam.
In stacking apparatus of the type described, the clamping beams can have very limited cross sectional areas because of the lack of available space. With very large machine widths, i.e. machine widths greater than 2000 mm, clamping beams which are supported only at their ends tend to bend between these ends. As a consequence, where a multiplicity of stacks are formed side by side and are clamped together, stacks in the middle cannot be reliably clamped and, in general, the clamping effect can be irregular and there can be damage to the sheets.
When an effort is made to solve this problem by increasing the clamping force, even greater bending can occur and, especially when the force at the sides is excessive, the clamping elements can be damaged and/or the sheets can be marked so that they become unusable or unmarketable.